What is CRISPR?

April 26, 2018

Genetic Defects can be treated thanks to the CRISPR tool.

Originally, CRISPR is an abbreviation of Clustered Regularly Interspaced Short Palindrome Repeats. This name was suggested years before the origin of interspacing subsequences. During those days, CRISPR was used to describe segments of prokaryotic DNA that had short and repetitive base sequences.

The advancement in technology led to a tool which has enough power to edit genes of an organism. The official name given to it was CRISPR-Cas9 but it is commonly known as CRISPR. Cas9 is an enzyme that has the ability to cut strands of DNA. That’s the reason why it is sometimes referred as ‘Molecular Scissor’ in the medical world.

A synthetic guide RNA (gRNA) is complexed with the Cas9 nuclease which is then inserted into the cell you want to treat. The cellular genome can be cut from any point which allows the removal of existing genes as well as the addition of new ones.

This Cas9-gRNA complex interacts with the DNA of the cell and breaks the bonds from where it is needed. The researchers have found several potential applications of this revolutionary tool given its ability to alter DNA sequences and modify the functionality of the genes. Some important applications include improvement of crops, correction of genetic defects, and prevention and treatment of spreading diseases. Some people are also trying to use CRISPR for germline editing.

All the insight for this technology came from the natural defensive mechanisms of bacteria and another single-cell organism called archaea. They use different proteins including Cas9 in order to counter attacks from viruses and other foreign bodies. Their most common mode of operation is the destruction of the DNA of an intruder. Researchers performed in-depth analysis of different organisms and decided to expand this amazing attribute to bigger and complex living beings.

When this attribute is repeated in multi-cellular organisms, their genes can be manipulated. There was no practical proof of this procedure before November last year. Hiroshi Nishimasu (University of Tokyo) and Mikihiro Shibata (Kanazawa University) led the team of researchers that published a paper in this context on 10th November 2017. They explained the working of a CRISPR and told the world what actually happens when this enzyme-accelerated activity takes place.

DNA sequences store encoded messages and instructions in them. Gene Editing is the name of a process where these specified sequences are disturbed in order to alter the stored message. For this sake, natural methods for DNA repair are changed according to the message we want. All the details about transforming natural CRISPR-Cas9 into an editing tool came from two separate research papers that were published in 2012.

These papers suggested that Cas9 can be controlled to cut any region of a given DNA. All you need to do is to change the nucleotide sequence of crRNA which will then bind to a complementary DNA. One of these papers that were written by Martin Jinek and his team further simplified the issue by creating a ‘Guide RNA’. It was formulated by fusing together a crRNA and a tracrRNA. This constitutes that guide RNA and Cas9 protein is the only requirements for editing a DNA.

A professor of Genetics at Harvard Medical School, George Church, explained the procedure in the following words:

“Operationally, you design a stretch of 20 [nucleotide] base pairs that match a gene that you want to edit. Then the RNA plus the protein [Cas9] will cut — like a pair of scissors — the DNA at that site, and ideally nowhere else.”

DNA starts to repair itself naturally once it is cut. This can happen in two different ways. One of them is ‘Non-homogeneous End Joining’ where both ends repair themselves and are joined together. This has a poor efficiency rate as a lot of errors can occur. One of them is a mutation which happens due to accidental insertion or deletion.

In the other method of repair, the gap is filled with a sequence of nucleotides. A strand of DNA is used as a template for this procedure which allows the researcher to add any information to a gene. The utility of CRISPR-Cas9 has increased many times in the last few years. The efficiency of this process has a huge role to play in this regard as acknowledged by Neville Sanjana of the New York Genome Center. He said,

“I think the public perception of CRISPR is very focused on the idea of using gene editing clinically to cure disease.”

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